1. Field of the Invention
The present invention relates to printers that print a full color image in a plurality of colors and more particularly to such color printer having a reduced size.
2. Background Art
Generally, thermal printers comprise a thermal head that has a heating device and a conveyance device that conveys a recording sheet and a film-like ink ribbon coated with an ink. In the thermal printer, the ink ribbon is loaded between the recording sheet and the thermal head such that an ink of the ink ribbon is transferred to the recording sheet by the thermal head that is driven in accordance with print data, thereby forming an image. Especially, in a color thermal printer, a multi-color ink ribbon with yellow, magenta and cyan inks is used to print a image sequentially in yellow, magenta cyan inks in an superimposed manner, thereby printing a full color image.
In the past, there are various thermal printers that print a color image with a multi-color ink ribbon. For example, Unexamined Japanese Patent Application KOKAI Publication No. H10-181122 discloses a printer (of a first type) that comprises conveyance rollers that convey a recording sheet along a conveyance path in one and the other directions and a thermal head disposed at a predetermined position in the vicinity of the conveyance path to print a full color image with a multi-color ink ribbon on a recording sheet. The printer of this type conveys the recording sheet in one direction while printing an image in one color ink of multi-color ink ribbon, and, when this printing ends, conveys the printing sheet in the opposite direction to the print start position. Such operation is repeated in each of the different inks of the ribbon, thereby forming a full color image.
Unexamined Japanese Patent Application KOKAI Publication No. H10-44556 discloses a printer (of a second type) which comprises a platen drum around which a recording sheet is wound, and a thermal head disposed at a predetermined position in the vicinity of the outer periphery of the platen drum. This printer causes the platen drum to make three rotations in a predetermined direction to print yellow, magenta and cyan images sequentially in this order in an superimposed manner on a recording sheet around the platen drum in the yellow, magenta and cyan inks contained in the ink ribbon.
Unexamined Japanese Patent Application KOKAI Publication No. H8-310067 discloses a printer (of a third type) which includes a ring-like conveyance path along which a recording sheet is conveyed, and a thermal head and a multi-color ink ribbon disposed outside the outer periphery of the conveyance path. In this printer, the recording sheet is conveyed in one direction in a circulating manner along the conveyance path with its recording surface facing the outside of the ring-like conveyance path. Each time the recording sheet passes through a printing position where the thermal head is disposed, an image is printed in a respective one of the different color inks contained in the ink ribbon in a superimposed manner on a previous image printed on the recording sheet. Such printing is repeated so as to form a full color image.
In the printer of the first type, each time one-color printing is terminated, the recording sheet is required to be conveyed back to the print start position to start to print a next image in a different color. Thus, it takes a relatively long time until a full color image has been printed.
In the printer of the second type, since the recording sheet is wound around the outer periphery of the platen drum, the platen drum is required to have such a relatively large diameter that the outer periphery of the drum corresponds in length to the recording sheet. In addition, the platen drum is required to be finished with high dimension accuracy, which increases the cost. Further, clamps are required which fix an end of the recording sheet to be wound around the outer periphery of the platen drum to the platen drum, which renders the structure complicated. Since the recoding sheet is clamped at one end to the platen drum, that end portion of the sheet does not contribute to the printing of the image or is wastefully used.
In the printer of the third type, the recording sheet is circulated in one direction and not conveyed between the two positions as in the printer of the first type, and hence the printing time is reduced. In the printer of the third type unlike that of the second type, neither a large-diameter platen drum whose outer periphery corresponds in length to the recording sheet nor clamps for the recording sheet are required to be provided. Thus, the printer of the third type is simplified in structure and reduced in cost compared to the printer of the second type.
In the printer of the third type, however, the thermal head, disposed outside the ring-like conveyance path, prints an image on the recording surface of the recording sheet that is conveyed along the conveyance path in a circulating manner with the recording face facing the outside of the conveyance path. Thus, in addition to the thermal head, the mechanism that moves the thermal head between its printing position and non-printing position, the ribbon cassette that contains the multi-color ink ribbon and the ribbon take-up shaft that takes up a used multi-color ink ribbon are disposed outside the ring-like conveyance path. This increases the size of the whole printer.